Press forming is a method of processing a steel sheet, which is the target of the forming, by pressing a die to the steel sheet and transferring a shape of the die to a blank. In the press forming, a reverse phenomenon of a strain due to elastic recovery, what is called springback, is generated by a residual stress in a press forming product as driving force after the press forming product is removed from the die, and thus a problem frequently arises in that the shape of the press forming product differs from a desired shape. In recent years, particularly mainly in the automobile industry, there is a strong tendency to use a high-strength steel sheet for automotive body parts from a viewpoint of weight saving of automotive bodies, thereby causing an increase in degree of the springback. The suppression of the springback becomes a critical issue for reducing development time and costs of automobiles.
It is effective to take actions of suppressing springback by identifying a portion having residual stress having caused the occurrence of the springback and the direction of the residual stress. Patent Literature 1 describes a technique that pays attention to the residual stress in a press forming simulation with a finite element method, changes the residual stress of part of the regions of a press forming product before a die release, evaluates a springback amount calculated in its state, determines how much the residual stress in the region affects the springback, and identifies the cause of the occurrence of the springback. In the technique described in Patent Literature 1, the physical quantities changed in the part of regions may include the sheet thickness, Young's modulus, and a plasticity coefficient besides the residual stress (refer to paragraph [0031]). The technique described in Patent Literature 1 cannot directly identify the portion having the residual stress having caused the occurrence of the springback and the direction of the residual stress as described above, but can find which physical quantity in which portion indirectly affects the springback as the cause of occurrence.